This invention relates generally to adhesive bonding and, more particularly, to a method for depositing controlled amounts of a thin, nearly monolayer adherent material onto a substrate.
In the fabrication of thin film circuits, noble metals generally serve as conductors for interconnecting devices on a common substrate. However, because of the generally poor adhesion of noble metals to ceramic or dielectric type substrates or layers, an adhesive layer is commonly required to anchor the noble metal to the underlying substrate or layer. In the prior art, the thickness of these adhesive layers has generally ranged from 100 to 1000 Angstrom units (.ANG.).
It is known that the use of adhesive layers in this range of thickness gives rise to a number of practical limitations. In many applications where a noble metal such as platinum may serve as a resistor or conductor, a typical adhesive layer of 100-1000.ANG. thickness increases the difficulty of achieving the optimum electrical properties of the designed resistor or conductor because of the subsequent diffusion or migration of the adhesive layer material into the conductive layer during stabilization or assembly processes, usually performed at elevated temperatures, typically between 300.degree. C. and 900.degree. C. In addition, where noble metals are used as conductors in high frequency microwave circuits, an adhesive layer of 100-1000.ANG. or greater may contribute to RF losses at these frequencies.
In U.S. Pat. No. 4,129,848, "Platinum Film Resistor Device," issued Dec. 12, 1978, to R. J. Frank and T. E. Salzer, the shortcomings of an adhesion layer were recognized, the cited shortcomings, however, referring to a relatively thick adhesion layer. The patent teaches a method of providing a silicon substrate having a silicon dioxide layer, sputter etching the layer of silicon dioxide to produce etch pits upon its exposed surface, sputter depositing a first quantity of platinum on the pitted surface of the silicon dioxide layer at a first power level for a first time period, and sputter depositing a second quantity of platinum over the first quantity at a reduced power level for a longer time period. The patent disclosure indicates that the pits or holes formed in the silicon dioxide layer enhances the adherence of the platinum film thereto.
Nevertheless, the present inventors have found that the above-described process has proved unsatisfactory in some circumstances. In particular, the adhesion between the deposited platinum and the pitted silicon dioxide layer has not been reliably maintained, especially following the extreme temperature conditions of an annealing step. It is believed to be necessary to provide an adhesive layer, such as titanium, between the substrate and the noble metal film, to provide optimum adhesion therebetween, yet an adhesion layer sufficiently thin that it does not significantly affect the electrical or thermal characteristics of the noble metal film.